Which perl are you using? Because I cannot reproduce this with AS1002.

Best guess: you are using cygwin which uses (c|z|ba|other)sh as the shell, but provides an executable or alias for dir?

c:\test>\Perl510\bin\perl.exe -v

This is perl, v5.10.0 built for MSWin32-x86-multi-thread
(with 3 registered patches, see perl -V for more detail)

Copyright 1987-2007, Larry Wall

Binary build 1002 [283697] provided by ActiveState http://www.ActiveSt
+ate.com
Built Jan 10 2008 11:00:53

[SNIP}

c:\test>\Perl510\bin\perl.exe -wle"print $]; system q[type junk.pl]";;
5.010000
use threads;
use strict;
use warnings;

print $], ' ', $threads::VERSION, "\n";

my $sub1 = sub { my $a=[ 1 .. 10000] };

my $m = 0;

for (;;) {
    printf "\r%d\t", $m++;
    my $thr = threads->new( $sub1 )->join;
    undef $thr;
}
__END__
c:\test>
[download]